There can be a lot of confusion and lack of planning in Percona XtraDB Clusters in regards to nodes becoming desynchronized for various reasons. This can happen a few ways:
- An IST or SST joining node catching up after a state transfer (Joined/Joining state)
- Using wsrep_desync for something like a backup
- Executing a rolling-schema-upgrade using using wsrep_OSU_method=RSU
When I say “desynchronized” I mean a node that is permitted to build up a potentially large wsrep_local_recv_queue while some operation is happening. For example a node taking a backup would set wsrep_desync=ON during the backup and potentially fall behind …
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